The present invention relates to an electrostatographic apparatus such as a copying machine comprising an improved development bias means.
In a typical electrostatic copying machine, a photoconductive drum is charged and radiated with a light image of an original document to form an electrostatic image through localized photoconduction. A developing means such as a magnetic brush applies a toner substance to the drum which develops the electrostatic image to produce a toner image. The toner image is transferred and fixed to a copy sheet to provide a permanent reproduction of the original document.
In order to prevent the white areas of the image from printing gray on the copy sheet, a bias voltage having the same polarity as the electrostatic image on the drum is applied to the magnetic brush. The magnitude of the bias voltage is on the order of the magnitude of the electrostatic image in the white image areas. By this means, toner particles are prevented from being attracted to the white image areas.
Due to current flow from the bias voltage source through the magnetic brush and toner substance to the drum, the bias voltage is not the controlling factor in the bias effect. The controlling factor is the bias current, which is dependent on the electrical resistance of the toner substance. Therefore, applying a predetermined bias voltage under all conditions will not produce correct development bias.
Where the toner substance comprises a magnetic carrier and toner particles made of a powdered resin, fatigue of the toner substance is produced by friction between the particles. A charge of a polarity opposite to that of the toner particles develops between the carrier particles after prolonged use. This charge reaches a saturation value and limits the amount of charge which can be produced on the toner particles. In addition, "spent toner particles" are produced in the toner substance which decrease the effective contact area between fresh toner particles and carrier particles and promote the formation of an opposite charge on the toner particles. A substantial proportion of spent toner particles in the toner substance increases the electrical resistance thereof since the effective contact area between the carrier particles is decreased. The increased toner resistance decreases the bias current at a specific value of applied bias voltage.
For this reason the bias effect cannot be properly produced by simply applying a predetermined bias voltage between the magnetic brush and the drum. It is furthermore not effective to provide a constant bias current by means of a constant current source, since this produces uneven copy density with certain image patterns.